I have grown and sold plants over the years. Growing things is labour intensive, sowing, pricking out, potting on. I have propagators too, plus two large heated benches. You need lots of shelving to store plants on, plus space to store bought in potting compost and pots. To be efficient and productive, you need to have an area where you can sow seeds and pot them on near where your staging or shelving is, it is very exhausting walking round with trays of plants, lugging bags of compost etc. Depending on what you are growing, you may need to fleece plants in a cold snap or in hot weather make sure there is adequate water and ventilation. What is your market? If you have a stall, you need to load a car, set up, take it all down, put it back in the car, put it all back and if it is bad weather you may make no money. Is there a shop/s that may take your stuff (sale and return? sometimes a bit iffy) so you can offload quantity. Ebay is quite a good place to sell plants, you have to make sure you have good packaging available as plants can suffer in the post. If you have a greenhouse, you could concentrate your efforts there to start with and see how you get on. Plug plants are always popular, people often forget to sow things at the right times, people always want some spring cabbages, nice lettuce plants in the spring. Cottage garden plants are very popular, if you know other gardeners you could could get stuff they are splitting and of course they don't need protection. One year, I bought lots of plug plants early on in the year, brought them on and then made them up into hanging baskets and tubs, but I did have buyers for them, all the traders at a local craft village wanted them for outside their shops. You could grow quite a bit in an 8x12 greenhouse, if you have sufficient staging and shelving to put everything on.